1. Field of the Invention
The present invention relates to a dielectric element applicable to capacitors, sensors, transducers, actuators and the like, piezoelectric element (electrostrictive element) using the same dielectric element, ink jet head, and method for producing the same head. The dielectric element is also suitable in particular for a piezoelectric element for ferroelectric memories, MEMS elements, memory heads, optical shutters and speakers.
2. Related Background Art
Dielectric materials of high relative dielectric constant have been in demand for capacitors. At the same time, films of ceramic materials, e.g., BaTiO3, have been becoming thinner to reduce their capacitor size. However, ceramic materials of BaTiO3, Pb (Zr, Ti) O3 or the like have a relative dielectric constant of 1500 or so at the highest. This, coupled with problems resulting from unsatisfactory sintering or defective structures in the interfaces, may damage characteristics of electronic devices in which the thin ceramic film is used. Nowadays, there is some movement to apply a PZT (111)-oriented film, which has a stable remanent polarization value, to memories. For example, Japanese Patent Application Laid-Open No. 2003-179278 discloses a method for producing a (111)-oriented film. This method first forms a YSZ (111)-oriented film as a buffer layer on a Si substrate, and then an SrRuO3 (SRO) (111)-oriented film by utilizing the lattices of the buffer layer, to form a (111)-oriented film on the Si substrate. This method, however, involves several problems. It needs a buffer layer, and also needs control of stress in the buffer layer, because it determines performance of the element. Furthermore, when a film is stress-controlled to have a tensile stress prevailing therein, the SRO (111)-oriented film may not be produced with stable crystallinity. Moreover, all of these films are formed by epitaxial growth and hence frequently with poor reproducibility. Therefore, there are demands for methods which can solve the above problems for producing uniaxially oriented films having the same orientation with uniform characteristics, or epitaxial film structures with high reproducibility.
Recently, studies have been extensively made to apply piezoelectric elements to MEMSs and other purposes, and a thin film of a piezoelectric element with excellent characteristics has been expected. A piezoelectric element is composed of a piezoelectric layer placed between electrodes to expand or contract when a voltage is applied to the layer, and is applicable to various purposes, e.g., motors, ultrasonic motors and actuators.
The materials used in the above-described applicable areas are PZT-based ones, discovered about 50 years ago. They are sintered at 1100° C. or higher, and have been developed by various methods, e.g., sol-gel, sputtering, MBE, PLD and CVD, when applied to thin-film elements. One of the major problems tending to occur in many cases when they are formed into thin films is physical destruction within the film or in the film interfaces. Therefore, attempts have been made to devise crystal structure of the piezoelectric layer to secure a high piezoelectric constant and voltage resistance. Japanese Patent Application Laid-Open No. H8-116103 discloses a sputtering-produced (001)-oriented film for ink jet heads. This method provides an oriented electrode on a substrate to control crystal structure of the piezoelectric layer. This method can form a (001)-oriented Pt electrode of high crystallinity on a single-crystal MgO substrate. However, applicability of the device is frequently limited, because a single-crystal MgO substrate is expensive and limited in size. Moreover, a (111)-oriented piezoelectric film is formed on the (111) plane of deliquescent MgO, and there is still a room for improvement left in the method for more stably forming a (111)-oriented Pt crystal film.